... I wouldn't know if I was looking at an xs1 gearbox, is there any significant indication that it's different, and which way are the gears closer, is first a lot longer or second lower, if that makes sense,
Hey, Jay. 70-71 1st gear was a bit taller than the later models. It puts 1st closer to 2nd, potentially closing the torque gap that exists there. In this XS1 chart, notice that the torque-vs-rpm graph is converted to thrust-vs-speed. The optimum shift points are at the line crossings.
Now, have a look at mrriggs charts in these threads:
http://www.xs650.com/threads/performance-more-power-and-higher-top-end.36563/
http://www.xs650.com/threads/gearing-for-your-best-performance.36605/
He's looking to close-up that 1st-2nd power drop. Of course, for this to have any meaning for your bike, you'd have to get your specific torque charts, then transpose them into a thrust-vs-speed chart, to see if you have a significant drop in 1st-to-2nd. A wide torque band in those rpms may make this change unnecessary. A narrow torque band in those rpms may indicate improvements with the early trans.
From what I've gathered from other info, swapping a trans should be drop-in, assuming you're swapping complete (original unmolested) both-shaft units. The kickstart is another issue, different toothcount to accomodate the 1st gear change. There was a change to the shiftdrum, but have no usable info on that.
I've been gathering tranny info from a variety of places, to put together a chart of some type that shows the changes over the years. A bit of a backburner project, this gets kinda complicated.
Things to consider in a 447 to 256 swap is the higher 1st gear (makes it a closer ratio), plus the slightly easier kickstart effort.
Here's the work-in-progress (as of 1-1-2015)
XS650 transmission evolution
*** Starting 1970 XS1, 256-parts
1st gear: 14t mainshaft, 31t countershaft, 26t kickstart
26/14 (1.857:1) kickstart ratio (See INXS thread on Kickstart)
*** Starting 1970 XS1, 256-parts, after s/n 001235
Change to mainshaft needle bearing OD, only cases changed.
*** Starting 1972 XS2, 306-parts
Change to 1st gear ratio
1st gear: 13t mainshaft, 32t countershaft, 25t kickstart
25/13 (1.923:1) kickstart ratio, harder to kick (See INXS thread on Kickstart)
Changes to shiftdrum, interchangability unknown. (to be done)
*** Starting 1977 XS650D, mid production at 1T3-721262
Changes to splines of both shafts and ALL gears.
Old shaft/splines have polished valley bottom.
New shaft/splines have polished spline tops.
5th gear shaft OD changed from 0.979" to 0.975" (uncertain)
5th gear wheel ID changed from 0.984" to 0.979" (uncertain)
Some changes to other parts, bearings and shims. (to be done)
*** Starting 1980 XS650G
(to be done)
*** MikesXS 5th gear change
5th gear wheel updated from 23t to 24t
Same gear diameter, same diametrical pitch, accomodated by slight change to depth of tooth valleys, involute gearface geometry remains unchanged.
Recommended to use his matching 05-0857 22t countershaft gear, possibly to match new gearface geometry.
You'd also have to change the sprocket(s) to make use of that taller 1st. That will upset your entire strategy with the other gears, maybe worse or better, depending on what's happening as you cross the traps...